I would say it depends on the height you are working with, Bob. For me vert becomes worth it when you can add canopy footprint to the fixed area you are working with. If it was me, I would be using the 600 and 400 stacked on top of each other in the 4x4 area. Keep your existing veg area and then you can always do something else in the 3x3 if you want to.
Things that I have found out in my vertical journey.
1/ plants like to be blown down upon. If you have fans blowing up into your space you will probably get wind burn on the leaves.
2/ get fresh air coming into the top and bottom of the cab. Especially if your using cool tubes and your filter is down below, which it often needs to be in a vert set up if, you are utilising the top part of the area for plants and light spread. Fresh air falling down onto your plants with fans blowing down is one of the best improvements I have made.
3/ Like normal growing, if you are restrcited by numbers, veg time pays off. Grow them big and keep them trained back against a net or gate/whatever to obtain max light spread. I,E don't let the plants smother your lights, this seriously effects yield.
4/ Having shelves: a previous poster complained that people where not growing vertical as they didn't have shelves with 4zillion small plants on them. No worries, each to their own. If you are going to create shelves and use multiple lights, ensure the medium that your plants on the shelves grow in, are well protected from the heat and glare from your lights. Not doing so seriously reduces root health, can often dry roots out, and is generally a ballache for the health and stamina of your ladies.
5/ Growing them big. You can create individual scrog screens vertically very easily. And the good thing is with vertical you can grow 6 foot tall plants if you have the height. Spread them out and watch as you get main colas growing out the middle of your plants
I am sure I'll add more in time. Any more Q's just let us know
Peace, DST